People need to be reminded that they attacked us, here in our country, and stopping the war elsewhere won't stop them from coming here and attacking us again.
By caving, the Spanish have set the US up for a round of pre-election terror events, in order to show how 'ineffective' the war has been.
The dems and dem-sympathetic press would jump on this in a heartbeat.
Although such acts would have the desperation of the Tet Offensive, the propaganda potential for the election (the joint dem/Al Qaida goal of defeating GWB) is immense. Just as Tet caused a shift in policy in the US, (even though the Viet Cong were pretty much eliminated as a fighting force--NVA were the main combatants afterward), The propaganda victory, aided and amplified by the US press, eventually cost 1.5 million South Vietnamese their lives.
In this case, however, we would not be able to hand the fight off to the indigenous residents, 'cause they are us.